462 Transactions. — Geology. 



Sir A. Geikie remarks*: "The conditions under which such 

 well-shaped idiomorphic crystals were formed were probably 

 different from those that governed the cooling and consolida- 

 tion of ordinary lavas." These crystals, however, were found 

 at a greater distance from the vents than is usual, since they 

 must be at least ten miles in a direct line from the Waitakerei 

 Range. 



The typical lava of the Cheltenham breccia is an andesite, 

 usually an augite-andesite, but hypersthene-andesite is also 

 present, as is the case at the Coromandel, a parallel line of 

 activity of much the same age. There is occasionally a 

 tendency to ophitic structure, but it is never pronounced, 

 and the ground-mass is typically hyalopilitic. The specific 

 gravity varies from 2-5 to 2-8, but in the majority of cases 

 does not exceed 2-7. This is the more remarkable since a 

 few of the rocks are basalts (but without olivine). This some- 

 what low specific gravity may be accounted for, however, by 

 several considerations. In the first place, there is frequently 

 a fair amount of dark-brown glass in the ground-mass. Teall 

 gives the specific gravity of andesites as ranging from 2-54 in 

 a glassy to 2-79 in a crystalline state. t In the second place, 

 some of the specimens are highly amygdaloidal, the amygdules 

 forming a large percentage of the rock. They generally con- 

 sist of chabasite, which has a specific gravity ranging from 

 2-06 to 217, so that the specific gravity of the whole rock 

 fragment would be much lowered. On the whole, then, they 

 may be taken as typical andesites, while a small percentage 

 are basalts without olivine. Except in one doubtful case I 

 have seen no olivine ; but a highly basic serpentine, containing 

 0-47 per cent, of nickel, has been described as occurring at 

 Manukau North Head, so that olivine-basalts may yet be 

 found in the lavas if not in the ejected fragments. 



Besides the more basic fragments there are others pre- 

 senting the appearance of true trachytes, pale-grey in colour, 

 with a specific gravity of 2'54. Occasionally fragments of 

 acid pumice are also present. It is possible that this did not 

 come from the Waitakerei vents. At the Tamaki Gulf, a few 

 miles from Auckland, there are pumice beds, which I con- 

 sider to be of Pliocene age. They lie unconformably on the 

 Waitemata series, and are due, I believe, to the fact that the 

 Waikato River then flowed into the Auckland sea. J Before 

 this, however, it flowed into the sea near Tauranga, in the 

 Bay of Plenty, and it cannot be supposed that it was flowing 

 into the Auckland sea so long ago as Miocene times. Except 



*" Ancient Volcanoes of Great Britain," vol. i., p. 62. 

 f " British Petrography." 

 J Trans. N.Z. Inst. 



